Crosslinking of photoiniator-initialized polyacrylates

ABSTRACT

A process for increasing the molecular weight of polyacrylates, characterized in that polyacrylates which are functionalized at least on one part of their chain ends by photoinitiator groups X are exposed to actinic radiation, so that a direct linking reaction of the polyacrylates takes place.

The invention relates to a process for increasing the molecular weightof polyacrylates and derivatives thereof, in particular forcrosslinking.

Among producers of acrylic pressure sensitive adhesives (PSAs) there isa trend toward reducing the proportion of solvent in the productionprocess. This relates in particular to the coating process, since herein general the polymers are coated from a solution with a concentrationof 20 or 30% onto the corresponding carrier material and subsequentlythe solvent is distilled off again in drying tunnels. As a result of theheat introduced, the drying step may additionally be utilized for thethermal crosslinking of the PSA.

If it is then desired to reduce the solvent fraction or to eliminate itcompletely, polyacrylate PSAs can be coated from the melt. This is doneat relatively high temperatures, since otherwise the flow viscositywould be too high and the adhesive would exhibit an extreme resilienceduring the coating operation. One example of a functioning commercialsystem is represented by the UV acResins™ from BASF AG. Here, a low flowviscosity at temperatures of less than 140° C. has been achieved bylowering the average molecular weight to below 300 000 g/mol.Accordingly, these materials are easy to coat from the melt. As a resultof the lowering, however, there is also a deterioration in the technicaladhesive properties, especially the cohesion, of these PSAs. Inprinciple, the cohesion can be raised by UV or EB crosslinking.Nevertheless, the UV acResins™ do not achieve the level of cohesionattained by high molecular mass acrylic PSAs which have been appliedconventionally from solution and crosslinked thermally.

A key problem is the network arc length, since acrylic hotmelt PSAsgenerally have a relatively low molecular weight, possess a relativelylow fraction of interloops, and thus need to be crosslinked to a greaterextent. Although the greater crosslinking does increase the level ofcohesion, the distance between the individual crosslinks becomes smallerand smaller. Consequently, the network is significantly tighter and thePSA then possesses only a low level of viscoelastic properties.

Accordingly, there is a need for a polymer which is easy to coat fromthe melt and is subsequently crosslinked on the carrier material in filmform in a specific way, so that, preferably, a linear polymer with onlyvery few crosslinking sites is formed.

Endgroup-functionalized polymers have already been known for a longtime. In U.S. Pat. No. 4,758,626, for example, polyesters were impactmodified using carboxy-terminated polyacrylates. However, no descriptionwas given there of specific endgroup crosslinking.

U.S. Pat. No. 4,699,950 describes thiol-functionalized polymers andblock copolymers. The polymers, however, contain only one functionalgroup, which is subsequently used for polymerization or for otherreaction.

U.S. Pat. No. 5,334,456 describes maleate- or fumarate-functionalizedpolyesters. Subsequent crosslinking takes place in the presence of vinylethers. Here again, polyacrylates are not described.

U.S. Pat. No. 5,888,644 describes a process for preparing releasecoating materials. Its starting point is formed by polyfunctionalacrylates, which are reacted with polysiloxanes. Here again, no definednetwork is formed, so that this process cannot be transferred either toacrylic PSAs.

U.S. Pat. No. 6,111,022 describes poly(meth)acrylonitrile polymersprepared by ATRP. Terminally functionalized polymers can also beprepared by these processes. Advantageous processes for preparingpurposively crosslinked PSAs are not disclosed, however.

In U.S. Pat. No. 6,143,848, terminally functionalized polymers areprepared by a new, controlled polymerization process. The polymerizationprocess employed is an iodine transfer process. However, polymers ofthis type lack great thermal stability, since iodides generally reactwith air and are easily oxidized to iodine. Severe discolorations are aconsequence of this. This applies in particular to hotmelt processeswith high temperatures.

None of the aforementioned documents points to a process in whichpolyacrylates endgroup-functionalized with photoinitiators have beendeliberately reacted therewith in order to construct a linear polymerchain or a polymer network.

It is an object of the invention to specify a process for building upthe molecular weight of polyacrylates, in particular for theircrosslinking, which has the disadvantages of the prior art only to areduced extent, if at all.

Surprisingly, and unforeseeably for the skilled worker, this object isachieved by the process of the invention, as specified in theindependent claim and in the subclaims.

The invention accordingly provides a process for increasing themolecular weight of polyacrylates, where polyacrylates functionalized atleast on one part of their chain ends by photoinitiator groups X (alsoreferred as functionalized groups below) are exposed to actinicradiation, so that a linking reaction of the polyacrylates takes place.

Here and below, the general term polyacrylates should also be understoodto include their derivatives, and polymethacrylates and theirderivatives.

With particular advantage the reaction in question is a direct linkingreaction, i.e., a linking reaction of the individual polyacrylatemolecules with one another.

It is likewise very advantageous as well, however, if the irradiationwith actinic radiation is conducted in the presence of at least onecrosslinker substance, so that a linking reaction of the polyacrylateswhich includes the crosslinker substance takes place.

In the case of the polyacrylates the photoinitiator groups X are locatedon their chain ends and are therefore also referred to below asfunctional end groups.

Very advantageously the polyacrylates functionalized with thephotoinitiator groups X have an average molecular weight (numberaverage) of Mn in the range from 2000 to 1 000 000 g/mol. Accordinglythe process is particularly suitable for the synthesis or for thecrosslinking of polyacrylate pressure sensitive adhesives.

By increasing the molecular weight is meant, in the sense of the processof the invention, in particular a crosslinking, but also, furthermore,the synthesis of higher molecular weight (longer-chain) molecules. Theprocess therefore allows the synthesis of higher molecular weightcompounds from the lower molecular weight components; in a version whichis particularly preferred for the process of the invention thecomponents (i.e., the polyacrylates containing photoinitiator group Xand, where appropriate, the crosslinker substances) are linked linearlyto one another.

In one preferred process version the polyacrylates used are composed ofat least 50% by weight of acrylic and/or methacrylic acid derivatives ofthe following general formula:

where R₁ is H or CH₃ and the radical R₂ is H or CH₃ or is selected fromthe group consisting of branched and unbranched saturated alkyl groupshaving 1 to 30, in particular 2 to 20 carbon atoms.

For polymerization the monomers are chosen such that the resultingpolymers can be used for pressure sensitive adhesives at roomtemperature or higher temperatures, particularly such that the resultingpolymers possess pressure sensitive adhesive properties in accordancewith the “Handbook of Pressure Sensitive Adhesive Technology” by DonatasSatas (van Nostrand, N.Y., 1989).

In order to obtain a preferred polymer glass transition temperatureT_(G)≦25° C., in accordance with the above remarks, the monomers arevery preferably selected in such a way, and the quantitative compositionof the monomer mixture advantageously chosen in such a way, that thepolymer is obtained with the desired T_(G) in accordance with the Foxequation (G1) (cf. T. G. Fox, Bull. Am. Phys. Soc. 1 (1956) 123).$\begin{matrix}{\frac{1}{T_{G}} = {\sum\limits_{n}\quad\frac{w_{n}}{T_{G,n}}}} & ({G1})\end{matrix}$

In this equation, n represents the serial number of the monomers used,w_(n) denotes the mass fraction of the respective monomer n (in % byweight), and T_(G,n) denotes the respective glass transition temperatureof the homopolymer of the respective monomers n, in K.

Preferably, use is made of acrylates and methacrylates having alkylgroups of 4 to 14 carbon atoms, preferably of 4 to 9 carbon atoms.Specific examples, without wishing to be restricted by this listing,include methyl acrylate, methyl methacrylate, ethyl acrylate, n-butylacrylate, n-butyl methacrylate, n-pentyl acrylate, n-hexyl acrylate,n-heptyl acrylate, n-octyl acrylate, n-octyl methacrylate, n-nonylacrylate, lauryl acrylate, stearyl acrylate, behenyl acrylate, and thebranched isomers thereof, such as isobutyl acrylate, 2-ethylhexylacrylate, 2-ethylhexyl methacrylate, isooctyl acrylate, and isooctylmethacrylate, for example.

Further classes of compounds which can be used include monofunctionalacrylates and methacrylates of bridged cycloalkyl alcohols, composed ofat least 6 carbon atoms. The cycloalkyl alcohols may also besubstituted, by C₁₋₆ alkyl, halogen or cyano, for example. Specificexamples include cyclohexyl methacrylates, isobornyl acrylate, isobornylmethacrylate, and 3,5-dimethyladamantyl acrylate.

Advantageously, monomers are used which carry polar groups such ascarboxyl, sulfonic and phosphonic acid, hydroxyl, lactam and lactone,N-substituted amide, N-substituted amine, carbamate, epoxy, thioliether, alkoxy, and cyano or the like.

Examples of moderate basic monomers are N,N-dialkyl-substituted amides,such as N,N-dimethylacrylamide, N,N-dimethylmethylmethacrylamide,N-tert-butylacrylamide, N-vinylpyrrolidone, N-vinyllactam,dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate,diethylaminoethyl methacrylate, diethylaminoethyl acrylate,N-methylol-methacrylamide, N-(butoxymethyl)methacrylamide,N-methylolacrylamide, N-(ethoxy-methyl)acrylamide,N-isopropylacrylamide, this list not being conclusive.

Further preferred examples are hydroxyethyl acrylate, hydroxypropylacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, allylalcohol, maleic anhydride, itaconic anhydride, itaconic acid, glyceridylmethacrylate, phenoxyethyl acrylate, phenoxyethyl methacrylate,2-butoxyethyl methacrylate, 2-butoxyethyl acrylate, cyanoethylmethacrylate, cyanoethyl acrylate, glyceryl methacrylate, 6-hydroxyhexylmethacrylate, vinylacetic acid, tetrahydrofurfuryl acrylate,α-acryloyloxypropionic acid, trichloroacrylic acid, fumaric acid,crotonic acid, aconitic acid, dimethylacrylic acid.

In another very preferred process variant, comonomers used include vinylesters, vinyl ethers, vinyl halides, vinylidene halides, and vinylcompounds with aromatic cycles and heterocycles in the a position. Hereagain, mention may be made nonexclusively of some examples: vinylacetate, vinylformamide, vinylpyridine, ethyl vinyl ether, vinylchloride, vinylidene chloride, and acrylonitrile.

Comonomers which possess a high static glass transition temperature arealso preferably added to the monomers described. Suitable here arearomatic vinyl compounds, such as styrene, in which case the aromaticnuclei are preferably composed of C₄ to C₁₈ units and may also containheteroatoms. Particularly preferred examples include 4-vinylpyridine,N-vinylphthalimide; methylstyrene, 3,4-dimethoxystyrene, 4-vinylbenzoicacid, benzyl acrylate, benzyl methacrylate, phenyl acrylate, phenylmethacrylate, t-butylphenyl acrylate, t-butylphenyl methacrylate,4-biphenylyl acrylate and methacrylate, 2-naphthyl acrylate andmethacrylate, and mixtures of those monomers.

The polyacrylates have on at least one part of their chain end in eachcase one photoinitiator group X. The photoinitiator systems may beidentical or else different photoinitiator systems within one moleculechain; furthermore, the photoinitiator systems of the individual polymermolecules can be chosen to be identical or different. Photoinitiatorsshould here be taken to be compounds which under actinic radiation format least one free radical.

As photoinitiators it is possible, for example, to use substances whichabsorb UV light. Some useful photoinitiators which are very good to useinclude benzoin ethers, such as benzoin methyl ether and benzoinisopropyl ether, for example, substituted acetophenones, such as2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenyl-1-phenylethanone,dimethoxyhydroxyacetophenone, substituted α-ketols, such as2-methoxy-2-hydroxypropiophenone, for example, aromatic sulfonylchlorides, such as 2-naphthylsulfonyl chloride, for example, andphotoactive oximes, such as 1-phenyl-1,2-propanedione2-(O-ethoxycarbonyl)oxime, for example.

The abovementioned photoinitiators X and others which can be used,including those of the Norrish I or Norrish II type, may contain thefollowing radicals: benzophenone, acetophenone, benzil, benzoin,hydroxyalkylphenone, phenyl cyclohexyl ketone, anthraquinone,trimethylbenzoylphosphine oxide, methylthiophenyl morpholine ketone,aminoketone, azobenzoin, thioxanthone, hexaarylbisimidazole, triazine,or fluorenone, it being possible for each of these radicals additionallyto be substituted by one or more halogen atoms and/or one or morealkyloxy groups and/or one or more amino groups or hydroxyl groups. Arepresentative overview is given by Fouassier: “Photoinitiation,Photopolymerization and Photocuring: Fundamentals and Applications”,Hanser-Verlag, Munich 1995. For further details, it is possible toconsult Carroy et al. in “Chemistry and Technology of UV and EBFormulation for Coatings, Inks and Paints”, Oldring (Ed.), 1994, SITA,London.

The abovementioned photoinitiators X may be attached at the respectiveend of the polymer chain by various means. Advantageously, thephotoinitiator X is attached to the chain end by the polymerizationprocess. In another advantageous procedure, the photoinitiator isattached selectively to the respective chain end by means of apolymer-analogous reaction. Also advantageously, the photoinitiator isgenerated on the respective chain end by a synthesis reaction.

Preferably, linear polyacrylates which each have one photoinitiator X onthe respective chain end are used for crosslinking. However, it is alsopossible, advantageously, for the linear polyacrylates to exhibitbranching along the polymer main chain, the side chains being able to beformed by the polymerization process; accordingly, with preference, thepolyacrylates functionalized with photoinitiator groups may contain atleast one chain branching site.

In one advantageous process version, two or more photoinitiators areattached in each case to one or more chain ends.

Moreover, in a further preferred procedure, specific branched polymersand star polymers are used. Advantageously, use is made of 3-arm, 4-arm,6-arm, 8-arm or 12-arm star polymers based on poly(meth)acrylate. It isalso possible to use hyperbranched polyacrylates. All polyacrylatescarry at least one photoinitiator X on the respective chain end.

In order to prepare the polyacrylates, controlled or conventionalfree-radical polymerizations will be carried out. For thepolymerizations proceeding by a radical mechanism it is preferred to useinitiator systems which additionally comprise further radical initiatorsfor the polymerization, especially thermally decomposing,radical-forming azo or peroxo initiators. In principle, however, anycustomary initiators that are familiar to the skilled worker foracrylates are suitable. The production of C-centered radicals isdescribed in Houben Weyl, Methoden der Organischen Chemie, Vol. E 19a,pp. 60-147. These methods are employed preferentially in analogy.

Examples of radical sources are peroxides, hydroperoxides, and azocompounds; some nonexclusive examples of typical radical initiators thatmay be mentioned here include potassium peroxodisulfate, dibenzoylperoxide, cumene hydroperoxide, cyclohexanone peroxide, di-t-butylperoxide, azodiisobutyronitrile, cyclohexylsulfonyl acetyl peroxide,diisopropyl percarbonate, t-butyl peroctoate, and benzpinacol.Preferably, 1,1′-azobis(cyclohexanecarbonitrile) (Vazo 88™ from DuPont)is used as radical initiator.

The average molecular weights (number average) M_(n) of the pressuresensitive adhesives formed in the course of the controlled radicalpolymerization chosen such as to be situated within a range from 2 000to 1 000 000 g/mol; specifically for further use as hotmelt pressuresensitive adhesives, PSAs having average molecular weights M_(n) of from100 000 to 500 000 g/mol are prepared. The average molecular weight isdetermined by size exclusion chromatography (gel permeationchromatography, SEC or GPC) or matrix-assisted laserdesorption/ionization mass spectrometry (MALDI-MS).

The polymerization may be carried out in bulk, in the presence of one ormore organic solvents, in the presence of water, or in mixtures oforganic solvents and water. The aim is to minimize the amount of solventused. Suitable organic solvents or solvent mixtures are pure alkanes(e.g., hexane, heptane, octane, isooctane), aromatic hydrocarbons (e.g.,benzene, toluene, xylene), esters (e.g., ethyl, propyl, butyl or hexylacetate), halogenated hydrocarbons (e.g., chlorobenzene), alkanols(e.g., methanol, ethanol, ethylene glycol, ethylene glycol monomethylether), and ethers (e.g., diethyl ether, dibutyl ether) or mixturesthereof. A water-miscible or hydrophilic cosolvent may be added to theaqueous polymerization reactions in order to ensure that in the courseof monomer conversion the reaction mixture is in the form of ahomogeneous phase. Cosolvents which can be used with advantage for thepresent invention are chosen from the following group, consisting ofaliphatic alcohols, glycols, ethers, glycol ethers, pyrrolidines,N-alkylpyrrolidinones, N-alkylpyrrolidones, polyethylene glycols,polypropylene glycols, amides, carboxylic acids and salts thereof,esters, organic sulfides, sulfoxides, sulfones, alcohol derivatives,hydroxy ether derivatives, amino alcohols, ketones, and the like, andalso derivatives and mixtures thereof.

The polymerization time is between 4 and 72 hours, depending onconversion and temperature. The higher the reaction temperature can bechosen, i.e., the higher the thermal stability of the reaction mixture,the lower the reaction time that can be chosen.

For the initiators which undergo thermal decomposition, the introductionof heat is essential to initiate the polymerization. For the thermallydecomposing initiators the polymerization can be initiated by heating atfrom 50 to 160° C., depending on initiator type.

For the preparation of polyacrylates it may also be of advantage toprepare the polymers in bulk, i.e., without solvent. In this case, aparticularly suitable techique is that of prepolymerization. Thepolymerization is initiated with UV light but conducted only to a lowconversion of about 10-30%. The resulting polymer syrup can then bewelded, for example, into films (in the simplest case, ice cubes) andthen polymerized in water to a high conversion. The resulting pelletscan then be used as acrylic hotmelt adhesives, in which case the filmmaterials used for the melting operation are with particular preferencematerials compatible with the poly(meth)acrylate.

Another possibly advantageous preparation process for the polyacrylatesis anionic polymerization. In this case it is preferred to use inertsolvents as the reaction medium, such as aliphatic and cycloaliphatichydrocarbons, for example, or else aromatic hydrocarbons.

In this case the living polymer is generally represented by thestructure P_(L)(A)-Me, in which Me is a metal from group 1, such aslithium, sodium or potassium, and P_(L)(A) is a growing polymer block ofthe monomers A. The molar mass of the endgroup-modifiedpoly(meth)acrylate to be prepared is controlled by the ratio ofinitiator concentration to monomer concentration. For the synthesis ofthe polymer it is preferred to use acrylate and methacrylate monomerswhich do not adversely affect, let alone cause complete termination of,the anionic polymerization process.

For the preparation of polyacrylates terminally functionalized with Xend photoinitiator groups, it may be of an advantage to add monomers forthe synthesis of a polymer block and then, by adding a second monomer,to attach a further polymer block containing the group X. Alternatively,suitable difunctional compound can be coupled. In this way it is alsopossible to obtain starblock copolymers (P(B)-P(A))_(n). In these cases,however, the anionic initiator ought already to carry the functionalgroup X, or the group ought to be obtainable by a subsequentpolymer-analogous reaction.

For general anionic polymerizations, examples of suitable polymerizationinitiators include n-propyllithium, n-butyllithium, sec-butyllithium,2-naphthyllithium, cyclohexyllithium, and octyllithium, with this listmaking no claim to completeness. Furthermore, initiators based onsamarium complexes are known for the polymerization of acrylates(Macromolecules, 1995, 28, 7886) and can be used here. With theseinitiators, however, it must be borne in mind that onlymono-endgroup-functional polyacrylates can be obtained by this route, byterminating the corresponding anionic polymerization. For thepreparation of carboxyl groups this can take place, for example, bymeans of CO₂ with subsequent hydrolysis; for the preparation of hydroxylgroups, for example, by reaction with ethylene oxide and subsequenthydrolysis. Functionalization by X is then performed by means of apolymer-analogous reaction with the hydroxyl group.

For the synthesis of the polyacrylates it is possible to usenitroxide-controlled polymerization processes. For the preferreddifunctional polyacrylates it is preferred to use difunctionalinitiators. One example of this is difunctional alkoxyamines

where R^(1*), R^(2*), R^(3*), and R^(4*) may be different, identical orchemically joined to one another and where the pairs R^(1*) and R²* andalso R³* and R^(4*) in each case contain at least one group X or possessa functional group which can be converted into X by chemical reaction.R¹ to R⁴ are preferably, independently of each other, chosen as:

-   i) halides, such as chlorine, bromine or iodine, for example,-   ii) linear, branched, cyclic and heterocyclic hydrocarbons having    from 1 to 20 carbon atoms, which may be saturated, unsaturated or    aromatic,-   iii) esters —COOR^(5*), alkoxides —OR^(6*) and/or phosphonates    —PO(OR^(7*))₂, where R⁵, R⁶′, and R⁷ stand for radicals from group    ii),-   iv) radicals from ii) where additionally at least one hydroxy    function or silyl ether function is present.

For the preparation of the terminally functionalized polyacrylates bynitroxide-controlled polymerization it is also possible to use further,different alkoxyamines. From the basic synthesis design, the middleblock, which following thermal initiation, initiation by thermalradiation or actinic radiation, forms two free radicals, can beadditionally varied or modified further. The skilled worker is aware ofa variety of chemical structures. The precondition is that at least twofree radicals are formed which are stabilized by nitroxides which carryat least one functional group X or a group which is converted into X bymeans of a chemical reaction.

In one favorable procedure, nitroxides of type (II) or (III) are usedfor radical stabilization:

where R^(1#), R^(2#), R^(3#), R^(4#), R^(5#), R^(6#), R^(7#); and R^(8*)independently of one another denote the following compounds or atoms andpreferably at least one of the radicals R^(1#) to R^(6#) or R^(7#)and/or R^(8#) carry at least one group X or contain a group which can beconverted into the desired group X by means of a chemical reaction.R^(1#) to R^(8#) are preferably, independently of each other, chosen as:

-   i) halides, such as chlorine, bromine or iodine, for example,-   ii) linear, branched, cyclic and heterocyclic hydrocarbons having    from 1 to 20 carbon atoms, which may be saturated, unsaturated or    aromatic,-   iii) esters —COOR^(9#), alkoxides —OR^(10#) and/or phosphonates    —PO(OR^(11#))₂, where R^(9#), R^(10#), and R^(11#) stand for    radicals from group ii),-   iv) radicals from the group ii) where additionally at least one    hydroxy function or silyl ether function is present.

Compounds of the above type (II) or (III) may also be attached topolymer chains of any kind (primarily such that at least one of theabovementioned radicals constitutes a polymer chain of this kind) andmay therefore be utilized, for example, for the synthesis ofend-functionalized polymers, as macroradicals or macroregulators.

Further general nitroxide-controlled processes for implementingcontrolled free-radical polymerizations are described below. U.S. Pat.No. 4,581,429 A discloses a controlled-growth radical polymerizationprocess which uses as its initiator a compound of the formula R′R″N—O—Y,in which Y denotes a free radical species which is able to polymerizeunsaturated monomers. WO 98/13392 A1 describes open-chain alkoxyaminecompounds which have a symmetrical substitution pattern. EP 735 052 A1discloses a process for preparing thermoplastic elastomers having narrowmolar mass distributions. WO 96/24620 A1 describes a polymerizationprocess in which very specific radical compounds, such asphosphorus-containing nitroxides based on imidazolidine, are used. WO98/44008 A1 discloses specific nitroxyls based on morpholines,piperazinones and piperazinediones. DE 199 49 352 A1 describesheterocyclic alkoxyamines as regulators in controlled-growth radicalpolymerizations. Corresponding further developments of the alkoxyaminesor of the corresponding free nitroxides improve the efficiency for thepreparation of polyacrylates (Hawker, contribution to the NationalMeeting of The American Chemical Society, Spring 1997; Husemann,contribution to the IUPAC World Polymer Meeting 1998, Gold Coast).

All of the abovementioned processes can be employed, by introducing oneor more functional groups X on the stabilizing nitroxide radical and/oron the polymerization-initiating radical, for preparingendgroup-functionalized polyacrylates.

A very preferred preparation process conducted is a variant of the RAFTpolymerization (reversible addition-fragmentation chain transferpolymerization). The polymerization process is described in detail, forexample, in the documents WO 98/01478 A1 and WO 99/31144 A1. Suitablewith particular advantage for the preparation of terminallyfunctionalized polyacrylates are trithiocarbonates of the generalstructure R′″—S—C(S)—S—R′″ (Macromolecules 2000, 33, 243-245), by meansof which one or more monomers (acrylates/methacrylates) are polymerizedand portions of the regulator remain as endgroups in the polymer. In thesimplest case, therefore, the trithiocarbonate may consist of onecompound, where R′″ contains a functional group X or a functional groupwhich can be converted into a functional group X by means of a chemicalreaction.

It may further be appropriate to carry out a two-stage polymerization.In a first step, monomers containing at least one functional group X arepolymerized using a trithiocarbonate and then used in a second step topolymerize the (meth)acrylates. The polymerization may take placecontinuously or with termination after the first stage, and subsequentreinitiation.

The latter method is particularly suitable for preparing terminallyfunctionalized polyacrylates containing two or more functional groups Xat each end.

Advantageously, use is made, for example, of the trithiocarbonates (IV)and (V) for the polymerization, with φ possibly being a phenyl ring,which is unfunctionalized or may be functionalized by alkyl or arylsubstituents linked directly or via ester or ether bridges, or a cyanogroup.

In order to promote the polymerization, the control, and the rate ofpolymerization it may be of advantage to use substituted compounds.Examples of possible functionalizations include halogens, hydroxylgroups, epoxy groups, groups containing nitrogen or groups containingsulfur, although this list makes no claim to completeness. Some of thesegroups may in turn be used as functional groups X.

Besides trithiocarbonates, however, it is also possible to use thefollowing structural units for the controlled polymerization, with kbeing defined as below:

In order to prepare terminally functionalized polyacrylates with fewgroups X or only one group X, on the other hand, it may be an advantageto use terminally functionalized trithiocarbonates. Particularlypreferably, use is made, for example, of trithiocarbonates of type VIIIand IX.

The group X ought not to influence the controlled free-radicalpolymerization. Moreover, the group k is highly variable, in order toimprove the control of the polymerization or to change thepolymerization rate. k can be C₁ to C₁₈ alkyl, C₂ to C₁₈ alkenyl, C₂ toC₁₈ alkynyl, in each case linear or branched; aryl, phenyl, benzyl,aliphatic and aromatic heterocycles. Furthermore, k can contain one ormore groups —NH₂, —NH—R^(VI), —NR^(VI)R^(VII), —NH—C(O)—R^(VI),—NR^(VI)—C(O)—R ^(VII), —NH—C(S)—R^(VI), —NR^(VI—C(S)—R) ^(VII),

where R^(VI) and R^(VII) can in turn be compounds of the type C₁ to C₁₈alkyl, C₂ to C₁₈ alkenyl, C₂ to C₁₈ alkynyl, in each case linear orbranched; aryl, phenyl, benzyl, aliphatic and aromatic heterocycles, andare independent of one another or the same.

It is, however, also possible to use regulators which carryfunctionalized dithioester groups at the end and which incorporate thesegroups at the end of the polymer. Regulators of this kind can in thesimplest case have the structure XII.

In this case, however, the functional group ought not to influence thepolymerization process but should instead remain on the sulfur atoms, sothat this group is incorporated at the end of the polymer chain.Furthermore, the dibenzylic group can be further modified and adapted inorder further to improve the polymerization properties. At this pointmention may be made, merely by way of example, of patents WO 98/01478 A1and WO 99/31144 A1.

As a further controlled polymerization method, atom transfer radicalpolymerization (ATRP) can be used advantageously to synthesize the blockcopolymers, in which case use is made preferably, as initiator, ofmonofunctional or difunctional secondary or tertiary halides and, forabstracting the halide(s), of complexes of Cu, Ni, Fe, Pd, Pt, Ru, Os,Rh, Co, Ir, Ag or Au (EP 0 824 111 A1; EP 826 698 A1; EP 824 110 A1; EP841 346 A1; EP 850 957 A1). The various possibilities of ATRP arefurther described in U.S. Pat. No. 5,945,491 A, U.S. Pat. No. 5,854,364A, and U.S. Pat. No. 5,789,487 A. For the preparation of the terminallyfunctionalized polyacrylates, the corresponding secondary or tertiaryhalide ought already to carry the desired functional group X. Moreover,as a result of the polymerization process, halides remain as endgroupsin the polymer, and may likewise be converted into the correspondingfunctional groups X by means of substitution reactions. In order toproduce multiblock or star-shaped structures it is possible to proceedin accordance with the design described in Macromolecules 1999, 32,231-234. There, polyfunctional halides are used for the polymerization,and must then be reacted further in a substitution reaction bypolymer-analogous means to give the desired functional group(s) X.

In order to produce multiarm, star-shaped or dendritic temrinalyfunctionalized polyacrylates endgroup-functionalized with X it islikewise possible to employ the polymerization processes describedabove. Through modification of the initiating compound or of theregulator, such compounds are readily available. The followingstructures show examples of suitable compounds, the compound XIII beinga suitable substance for preparing a 12-arm polyacrylate by an ATRPtechnique, the compound XIV being suitable for preparing a 6-armpolyacrylate by a RAFT technique, and the compound XV being suitable forpreparing a 3-arm polyacrylate via nitroxide-controlled reaction.

The abovementioned examples are intended only to be exemplary in nature.Polyacrylates prepared from compound Xlii can be converted, for example,by reaction (substitution reaction) of the terminal bromides intosuitable endgroup-functionalized polyacrylates. Polyacrylates preparedfrom compound XIV already possess one functional group X per polymer armas endgroup. The regulator XIV may, however, also carry this functionalgroup at another position on the terminal phenyl rings or else may carrytwo or more functional groups on the terminal phenyl rings.Polyacrylates prepared from compound XV already possesses 3-hydroxylgroups per polymer arm as terminal functional groups, which can be usedfor the generation of X.

The number of arms produced can be controlled by the number of thegroups which are essential for the controlled free-radicalpolymerization. Moreover, it is also possible to exchange, modify ortargetedly substitute functional groups. By means of this measure it ispossible, for example, to increase or lower the control or the rate ofpolymerization. Furthermore, all of the abovementioned polymerizationmethods depict only exemplary compounds for preparing polyacrylatesterminally functionalized with photoinitiator groups X. It is alsopossible, however, to employ all of the methods of controlledpolymerization that are familiar to the skilled worker, provided thesepolymerization methods allow the introduction of functional groups X atthe polymer end.

Besides the controlled radical methods, further free-radicalpolymerization methods are also suitable for introducing functionalgroups. By way of example mention may be made merely of thiol-regulatedcompounds, in which case the thiols or dithio compounds may likewisecarry functional groups X and thus effect terminal functionalization ofpolyacrylates. Furthermore, functional groups can be introduced into thepolymer as endgroups by means of the initiator. There exist, forexample, commercial azo initiators, which carry free carboxylic acidgroups or hydroxyl groups, which then, likewise by way of thepolymerization, can be installed in the polymer at the ends and utilizedfor the coupling reaction. Another possibility would be to scavenge thefree radical polymerization and in that way incorporate a functionalgroup X.

For further development, resins may be admixed to the polyacrylates. Astackifying resins for addition it is possible without exception to useany tackifier resins which are known and are described in theliterature. As representatives, mention may be made of pinene resins,indene resins, and rosins, their disproportionated, hydrogenated,polymerized, esterified derivatives and salts, the aliphatic andaromatic hydrocarbon resins, terpene resins and terpene-phenolic resins,and also C5, C9, and other hydrocarbon resins. Any desired combinationsof these and other resins may be used in order to adjust the propertiesof the resulting adhesive in accordance with what is desired. In generalit is possible to use any resin which is compatible (soluble) with thecorresponding polyacrylate; in particular, reference may be made to allaliphatic, aromatic, and alkylaromatic hydrocarbon resins, hydrocarbonresins based on pure monomers, hydrogenated hydrocarbon resins,functional hydrocarbon resins, and natural resins. Express reference ismade to the depiction of the state of the art in the “Handbook ofPressure Sensitive Adhesive Technology” by Donatas Satas (van Nostrand,1989).

Furthermore, it is also possible optionally to add plasticizers, fillers(e.g., fibers, carbon black, zinc oxide, titanium dioxide, chalk, solidor hollow glass beads, microbeads made of other materials, silica,silicates), nucleators, blowing agents, compounding agents and/or aginginhibitors, in the form for example of primary and secondaryantioxidants or in the form of light stabilizers.

In order to produce hotmelt poly(meth)acrylate PSAs, the polymersdescribed above are preferably coated as hotmelt systems. For theproduction process it may therefore be necessary to remove the solventfrom the PSA. In principle it is possible here to use any of thetechniques known to the skilled worker. One very preferred technique isthat of concentration using a single-screw or twin-screw extruder. Thetwin-screw extruder may be operated corotatingly or counterrotatingly.The solvent or water is distilled off preferably by way of severalvacuum stages. Moreover, counterheating is carried out depending on thedistillation temperature of the solvent. The residual solvent fractionsare preferably <1%, more preferably <0.5% and very preferably <0.2%. Thehotmelt is processed further from the melt.

In one very preferred procedure, the crosslinking reaction is promotedby adding vinyl compounds with a functionality of at least two. In onefurther very preferred procedure, for example, trifunctional vinylcompounds are added. An example of the crosslinking principle isdepicted in the following diagram:

where B, as a linking unit, can be an organic compound, an oligomer or apolymer.

Preferably, the component to be added is an organic compound containingat least two unsaturated groups. For increasing the rate of crosslinkingfurther, difunctional or polyfunctional methacrylates are preferred todifunctional or polyfunctional terminal vinyl compounds. Verypreferably, difunctional or polyfunctional acrylates are used for thecoupling reaction.

Besides the low molecular mass organic compounds, it is also possible touse higher molecular mass compounds (oligomers) or polymers ascrosslinker substances with at least 2 vinyl compounds. The polymergroup may embrace, for example, polyacrylates, polymethacrylates,polyisobutene, polyethylene, polypropylene, polyvinyl acetate,polyurethane, polyvinyl chloride, polystyrene, polycaprolactam,polycaprolactone, polyesters, polybenzoates, polysiloxanes,polyethylene/propylene copolymers, polybutadiene, polyisoprene,polybutene, polythiophene, polyacetylene, polyanthracene, polysilanes,polyamides, polycarbonates, polyvinyl alcohol, polypropylene oxide,polyethylene oxide, polyphenylene, polychloroprenes, and fluorinatedpolymers.

Furthermore, it is also possible in principle to add further,unattached, UV-absorbing photoinitiators. Useful photoinitiators whichare very good to use include benzoin ethers, such as benzoin methylether and benzoin isopropyl ether, for example, substitutedacetophenones, such as 2,2-diethoxyacetophenone (available as Irgacure651® from Ciba Geigy®), 2,2-dimethoxy-2-phenyl-1-phenylethanone,dimethoxyhydroxyacetophenone, substituted α-ketols, such as2-methoxy-2-hydroxypropiophenone, for example, aromatic sulfonylchlorides, such as 2-naphthylsulfonyl chloride, for example, andphotoactive oximes, such as 1-phenyl-1,2-propanedione2-(O-ethoxycarbonyl)oxime, for example.

The abovementioned photoinitiators and others which can be used,including those of the Norrish I or Norrish II type, may contain thefollowing radicals: benzophenone, acetophenone, benzil, benzoin,hydroxyalkylphenone, phenyl cyclohexyl ketone, anthraquinone,trimethylbenzoylphosphine oxide, methylthiophenyl morpholine ketone,aminoketone, azobenzoin, thioxanthone, hexaarylbisimidazole, triazine,or fluorenone, it being possible for each of these radicals additionallyto be substituted by one or more halogen atoms and/or one or morealkyloxy groups and/or one or more amino groups or hydroxyl groups. Arepresentative overview is given by Fouassier: “Photoinitiation,Photopolymerization and Photocuring: Fundamentals and Applications”,Hanser-Verlag, Munich 1995. For further details, it is possible toconsult Carroy et al. in “Chemistry and Technology of UV and EBFormulation for Coatings, Inks and Paints”, Oldring (Ed.), 1994, SITA,London.

For the very preferred use of the polyacrylates terminallyfunctionalized with photoinitiator groups X, they are coated either fromsolution or from the melt onto a carrier material.

Carrier materials used in the case of use as a pressure sensitiveadhesive, for PSA tapes, for example, are the materials which arecustomary and familiar to the skilled worker, such as films (polyesters,PET, PE, PP, BOPP, PVC), nonwovens, foams, wovens, and woven sheets, andalso release paper (glassine, HDPE, LDPE). This list is not conclusive.

UV crosslinking is effected by brief irradiation with ultravioletradiation in a wavelength range from 200 to 400 nm, depending on the UVphotoinitiator X used, especially using high or medium pressure mercurylamps with an output of from 80 to 240 W/cm. The irradiation intensityis adapted to the respective quantum yield of the UV photoinitiator andthe degree of crosslinking to be brought about.

A further option is to crosslink the applied polyacrylates, terminallyfunctionalized with photoinitiator groups X, with electron beams.Typical irradiation equipment which may be used includes linear cathodesystems, scanner systems, and segmented cathode systems, where electronbeam accelerators are concerned. A detailed description of the state ofthe art, and the most important process parameters, can be found inSkelhorne, Electron Beam Processing, in Chemistry and Technology of UVand EB formulation for Coatings, Inks and Paints, Vol. 1, 1991, SITA,London. The typical acceleration voltages are situated in the rangebetween 50 kV and 500 kV, preferably between 80 kV and 300 kV. Thescatter doses employed range between 5 and 150 kGy, in particularbetween 20 and 100 kGy.

Besides the inventive process for coupling and crosslinkingpolyacrylates, part of this invention is the use of such systems forpressure sensitive adhesive tapes, in particular for single-sided ordouble-sided PSA tapes.

Depending on polymer composition, the polymers composed or crosslinkedas described above and the processes for crosslinking them may also beutilized for films or release coating materials, Highly halogenatedpolymers could also be used, for example, as flame retardants. Moreover,it is also possible to use the inventive process to prepareheat-activatable PSAs. For this application, the polyacrylate terminallyfunctionalized with photoinitiator groups X ought to possess a glasstransition temperature of more than 25° C. For the polymers of theinvention with a close polymer network, applications in the coatingsfield are also possible. Polymers having a high glass transitiontemperature, prepared by the inventive process, may likewise be employedas thermoplastics.

Test Methods

A. Shear Stability Times

The test took place in accordance with PSTC-7. A 50 μm thick pressuresensitive adhesive layer is applied to a 25 μm thick PET film. A stripof this sample 1.3 cm wide is bonded to a polished steel plate over alength of 2 cm, by rolling over it back and forth three times using a 2kg roller. The plates are equilibrated for 30 minutes under testconditions (temperature and humidity) but without loading. Then the testweight is hung on, exerting a shearing stress parallel to the bondsurface, and the time taken for the bond to fail is measured. If aholding time of 10000 minutes is reached, the test is terminated beforethe adhesive bond fails.

B. Bond Strength

The testing of the peel adhesion (bond strength) took place inaccordance with PSTC-1. A 50 μm thick pressure sensitive adhesive layeris applied to a 25 μm thick PET film. A strip of this sample 2 cm wideis bonded to a steel plate by rolling back and forth over it three timesusing a 2 kg roller. The steel plate is clamped in and the self-adhesivestrip is pulled off from its free end at a peel angle of 180° using atensile testing machine.

C. Gel Permeation Chromatography GPC

The average molecular weight M_(w) and the polydispersity PD weredetermined by the company Polymer Standards Service of Mainz, Germany.The eluent used was THF containing 0.1% by volume trifluoroacetic acid.Measurement was made at 25° C. The precolumn used was PSS-SDV, 5μ, 10³Å, ID 8.0 mm×50 mm. Separation was carried out using the columnsPSS-SDV, 5μ, 10³ and also 10⁵ and 10⁶ each of ID 8.0 mm×300 mm. Thesample concentration was 4 g/l and the flow rate 1.0 ml per minute.Measurement was made against PMMA standards.

D. Gel Fraction

The carefully dried, solvent-free adhesive samples are welded into apouch of polyethylene nonwoven (Tyvek nonwoven). The gel index, i.e.,the toluene-insoluble weight fraction of the polymer, is determined fromthe difference in the sample weights before and after extraction withtoluene.

Amine-Functionalized UV Photoinitiator:

3-[4-(Dimethylamino)phenyl]-1-[4-(2-hydroxyethoxy)phenyl]-2-propen-1-one(XVI):

A mixture of 15 g of 2-bromoethanol, 16.3 g of p-hydroxyacetophenone and5.3 g of sodium hydroxide in 100 ml of dimethylformamide (DMF) washeated at 150° C. for 15 hours. The mixture was then poured into waterand the product was extracted with dichloromethane. Subsequent vacuumdistillation gave 11.4 g of a white solid(4-(2-hydroxyethoxy)acetophenone).

In a second reaction, a mixture of 8.3 g of p-dimethylaminobenzaldehyde,10.0 g of 4-(2-hydroxyethoxy)acetophenone and 2.5 g of sodium hydroxidein 100 ml of methanol was heated at reflux for 10 hours. The reactionmixture was then cooled using an ice bath and filtered and the solidisolated by filtration was washed with cold methanol. The product wasthen dried in a vacuum drying cabinet at 40° C. and 10 torr. 10.2 g ofwhite solid were isolated. The melting point was 128° C. (cf. U.S. Pat.No. 4,565,769, m.p.: 127-128.5° C.)

Preparation of a RAFT Regulator:

The regulator bis-2,2′-phenylethyl trithiocarbonate was preparedstarting from 2-phenylethyl bromide using carbon disulfide and sodiumhydroxide in accordance with the set of instructions in Synth. Comm.,1988, 18 (13), 1531. Yield: 72%. ¹H-NMR (CDCl₃), δ:7.20-7.40 ppm (m,10H); 3.81 ppm (m, 1H); 3.71 ppm (m, 1H); 1.59 ppm (d, 3H); 1.53 ppm (d,3H).

EXAMPLE 1

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 40 g of benzoin acrylate, 0.4 g of bis-2,2′-phenylethyltrithiocarbonate and 100 g of acetone/isopropanol (95/5). The batch wasrendered inert with nitrogen gas while being stirred with an anchorstirrer at room temperature for 1 hour. It was then heated to aninternal temperature of 58° C. using an oil bath, after which 0.2 g ofVazo 64™ (from DuPont) (2,2′-azobis(isobutyronitrile)) in solution in 5g of acetone was added. After a polymerization time of 24 hours, thebatch was cooled to room temperature and acetone was distilled off on arotary evaporator. Analysis by GPC (test C, PMMA standards) gave amolecular weight of M_(w) of 3 250 g/mol and M_(w) of 7 980 g/mol.

Subsequently this oligomeric polyacrylate, 300 g of 2-ethylhexylacrylate and 60 g of acrylic acid were dissolved in 150 g of acetone andthe solution was rendered inert using nitrogen gas for 1 hour and thenheated to an internal temperature of 58° C. again. At this temperature,0.2 g of Vazo 64™ (DuPont) (2,2′-azobis(isobutyronitrile)) in solutionin 5 g of acetone was added. The polymerization was conducted at aconstant external temperature of 70° C. Following a reaction time of 6hours, the batch was diluted with 100 g of acetone. After a reactiontime of 24 hours, a further 0.2 g of Vazo 64™ (DuPont)(2,2′-azobis(isobutyronitrile)) in solution in 5 g of acetone was added.After 30 hours the batch was diluted with 50 g of acetone. Thepolymerization was terminated by cooling to room temperature after areaction time of 48 hours. Analysis by GPC (test C, PMMA standards) gavea molecular weight M_(n) of 146 000 g/mol and M_(w) of 356 000 g/mol.

Thereafter the solvent was removed in a drying cabinet at 60° C. under avacuum of 10 torr. After that, 1% by weight of pentaerythritoltriacrylate was mixed into the melt and the composition was applied fromthe melt at 150° C. to a primed PET film 23 μm thick. The applicationrate was 50 g/m². In order to activate the coupling reaction the PSAspecimen was irradiated in a number of passes with a medium pressuremercury lamp (120 W/cm) from Eltosch with a belt speed of 20 m/min. Thetechnical adhesive properties were tested by carrying out test methods Aand B.

EXAMPLE 2

A 2 L glass reactor conventional for free-radical polymerizations wascharged with 40 g of acrylic acid, 0.4 g of bis-2,2′-phenylethyltrithiocarbonate and 160 g of DMF. The batch was rendered inert withnitrogen gas while being stirred with an anchor stirrer at roomtemperature for 1 hour. It was then heated to an internal temperature of58° C. using an oil bath, after which 0.2 g of Vazo 64™ (from DuPont)(2,2′-azobis(isobutyronitrile)) in solution in 5 g of DMF was added.After a polymerization time of 24 hours, the batch was cooled to roomtemperature and DMF was distilled off on a rotary evaporator. Analysisby GPC (test C, PMMA standards) gave a molecular weight of Mn of 2 820g/mol and M_(w) of 7 540 g/mol.

Subsequently this oligomeric polyacrylate, 300 g of 2-ethylhexylacrylate and 60 g of methyl acrylate were dissolved in 150 g ofacetone/n-butanol (7:3) and the solution was rendered inert usingnitrogen gas for 1 hour and then heated to an internal temperature of58° C. again. At this temperature, 0.2 g of Vazo 64™ (DuPont)(2,2′-azobis-(isobutyronitrile)) in solution in 5 g of acetone wasadded. The polymerization was conducted at a constant externaltemperature of 70° C. Following a reaction time of 6 hours, the batchwas diluted with 80 g of acetone. After a reaction time of 24 hours, afurther 0.2 g of Vazo 64™ (DuPont) (2,2′-azobis(isobutyronitrile)) insolution in 5 g of acetone was added. After 30 hours the batch wasdiluted with 50 g of acetone. The polymerization was terminated bycooling to room temperature after a reaction time of 48 hours. Analysisby GPC (test C, PMMA standards) gave a molecular weight Mn of 166 000g/mol and M_(w) of 421 000 g/mol.

The resulting polymer was then concentrated in the presence of 0.5% byweight of3-[4-(dimethylamino)phenyl]-1-[4-(2-hydroxyethoxy)phenyl]-2-propen-1-one,again based on the polymer. Thereafter the solvent was removed in adrying cabinet at 60° C. under a vacuum of 10 torr. After that, 1% byweight of pentaerythritol triacrylate was added in a hotmelt process(see above) and the composition was applied from the melt at 140° C. toa primed PET film 23 μm thick. The application rate was 50 g/m². Inorder to activate the coupling reaction the PSA specimen was irradiatedin a number of passes with a medium pressure mercury lamp (120 W/cm)from Eltosch with a belt speed of 20 m/min. The technical adhesiveproperties were tested by carrying out test methods A and B.

Results:

In example 1, using a trithiocarbonate regulator, a polyacrylate wasprepared with a plurality of benzoin units at the respective polymerchain end. The polymer was subsequently freed from the solvent, mixedwith a polyfunctional acrylate, and coated from the melt. After thecoating operation, the composition was UV-irradiated using a mediumpressure mercury lamp and was crosslinked directly on the carrier.

In example 2, a polymer provided with UV photoinitiators was coated fromthe melt and then UV-crosslinked. The polymer was reacted with anamine-functionalized UV photo-initiator in a polymer-analogous reaction.The reaction led likewise to a polyacrylate containing a plurality ofphotoinitiator molecules, attached to the polymer chain end. Thispolymer was blended with a polyfunctional acrylate and, followingcoating from the melt was UV-crosslinked on the carrier.

The table below summarizes the technical adhesive data, including thedata for the case of actinic radiation where appropriate. From thetable, however, it is very clearly evident that the process of theinvention is highly suitable for preparing pressure sensitive adhesives.UV dose Gel fraction SST 23° C., 10 N BS steel Material [mJ/cm²] [%][min] [N/cm] Example 1 UV-C: 100 59 4 780 3.7 UV-B: 560 Example 1 UV-C:150 70 8 725 3.5 UV-B: 840 Example 2 UV-C: 100 52 1 420 4.2 UV-B: 560Example 2 UV-C: 150 65 1 810 4.0 UV-B: 840Application rate: 50 g/m²BS: immediate bond strength on steelSST: shear stability timesUV dose: measured with the Power-Puck ™ from Eltosch

1. A process for increasing the molecular weight of polyacrylates,wherein polyacrylates which are functionalized at least on one part oftheir chain ends by photoinitiator groups X are exposed to actinicradiation, so that a direct linking reaction of the polyacrylates takesplace.
 2. A process for increasing the molecular weight ofpolyacrylates, wherein polyacrylates which are functionalized at leaston one part of their chain ends by suitable photoinitiator groups X areexposed to actinic radiation in the presence of at least one crosslinkersubstance, so that a linking reaction of the polyacrylates whichincludes the crosslinker substance takes place.
 3. A process accordingto claim 1 or 2, wherein the polyacrylates functionalized withphotoinitiator groups have an average molecular weight (number average)M_(n) in the range from 2 000 to 1 000 000 g/mol.
 4. A process accordingto claim 1 or 2, wherein the photoinitiator groups are those of UVphotoinitiators of the Norrish I and/or Norrish II type.
 5. A processaccording to claim 1 or 2, wherein the increase in the molecular weightcomprises a crosslinking reaction.
 6. A process as claimed in claim 1 or2, wherein the linking reactions link the polyacrylate moleculeslinearly to one another.
 7. A process as claimed in claim 1 or 2,wherein the polyacrylates functionalized with photoinitiator groupscontain at least one chain branching site.
 8. A process as claimed inclaim 7, wherein the polyacrylates have at least three photoinitiatorgroups.
 9. A process as claimed in claim 1 or 2, wherein at least onedifunctional or polyfunctional vinyl compound is added to thepolyacrylate functionalized with photoinitiator groups.
 10. A pressuresensitive adhesive for single-sided and double-sided pressure sensitiveadhesive tapes comprising at least one Polyacrylate prepared by theprocess of claim 1 or
 2. 11. A process according to claim 9, whereinsaid vinyl compound is a methacrylate-based compound.
 12. A processaccording to claim 9, wherein said vinyl compound is an acrylate-basedcompound.